Recently, considerable attention has been paid to energy conservation in buildings, particularly domestic housing. Both the public and the housing industry have become aware of the desirability of energy conservation. Consequently, new standards of energy efficient housing designs and construction are being developed. These standards contain stringent energy efficient requirements, with respect to thermal insulation, air tight construction, space heating, space ventilation, water heating, controlled air management and passive solar design features.
This has led to the development of the R-2000 house. The amount of energy used by space heating systems in R-2000 houses dropped to as little as 1/10 of the energy used in conventional housing units. In some energy efficient designs, the demand for supplementary heat may be so low that a furnace becomes unnecessary.
Another somewhat surprising consequence is that, since the heating requirements are considerably reduced, there is little incentive to utilize or design highly efficient furnaces for such housing. The additional cost in making the furnace or space heating unit efficient is frequently considerable, but since the heating costs are in any event low, the economic savings from reduced fuel consumption may not be justified by the increased cost of the initial installation.
One way in which housing is made energy efficient is to make the house extremely air tight, so as to reduce leakage of heated air. However, it is now recognized that in any air tight house, provision must be made for a continuous, controlled exchange of indoor air with outdoor air, to avoid potential health and house damage problems from inadequate ventilation. In a proposed revision of the National Building Code of Canada (B. H. Dickens, "Controlled Ventilation and Housing", A Summary Review, 008-T5, Energy, Mines & Resources Canada, Dec., 1985), it is specified that a mechanical ventilation system must be provided, whether or not provision is also made for natural ventilation. In other words, reliance on natural ventilation is no longer acceptable. It is required that the mechanical ventilation system must have a minimum capacity of 0.5 ACH (Air Changes per Hour) for houses where natural ventilation for summer cooling is provided. In the absence of the capability for summer cooling, the capacity is required to be 1.0 ACH. The mechanical ventilation is required to be independent of any natural ventilation sources such as infiltration and flow through windows.
Currently, for energy efficient or R-2000 houses, the space conditioning services are provided by conventional space heating systems coupled with conventional ventilating systems and air-to-air heat recovery systems.
A variety of space heating systems are employed. For central systems, the energy source may be natural gas, oil or electric power, and the furnace may be conventional, high efficiency or an ultrahigh efficiency condensing type.
The ventilation of an R-2000 home is usually combined with recovery of heat from the exhausted air in a standard air-to-air heat exchanger equipped with defrost controls. At present, various types of air-to-air heat exchangers are used, with various effectiveness ratings. In a Canadian winter, at temperatures below freezing (0.degree. C. to -20.degree. C.), evaluation of various systems has shown the following disadvantages and loss of effectiveness:
(1) In solid core systems, of various materials, and both counterflow or double crossflow, severe frost buildup can cause up to a 50% drop in sensible heat recovery effectiveness and up to a 16% leakage of exhausted air into the fresh air supply and a significant drop in air delivery capacities;
(2) For current heat wheel concepts, severe frost buildup reduces sensible heat recovery effectiveness up to 22%, air delivery capacities are reduced by up to 34% and the leakage of the exhaust air into fresh air is up to 30%;
(3) For current heat pipe systems, measured sensible heat recovery effectiveness was in the range from 47-55%.
A major problem with existing air-to-air heat exchangers is the buildup of ice on the heat transfer surfaces affecting the heat transfer efficiency and air flow capacity. This depends on air stream temperatures and humidities, together with duration of the period of freezing and the system's design. For current air-to-air heat recovery systems, buildup of ice starts at temperatures below 26.degree. F., although options for control of freeze-protection systems have been reported.
If one considers the requirements for space heating and ventilation of R-2000 houses, due to the high degree of their energy efficiency, the heating requirements are low. Thus, by way of example, for a 155 m.sup.2 typical two storey house in a 6000 D.D. zone with 150 heating days per year, the average heat consumption would be 7,347 BTU/h. If one assumes a gas fired furnace which is operated for 50% of the time, then the required output of the furnace would be 14,695 BTU/h (which is equivalent to 94.8 BTU/h/m.sup.2).
The guidelines for R-2000 houses further specify a ventilation requirement of a minimum of 0.45 ACH, which is to be provided by mechanical means. For a 155 m.sup.2 house, the ventilating capacity would then be 102 SCFM. If one assumes a 70% effective air-to-air heat recovery system, for a 75.degree. F. indoor temperature and a 0.degree. F. outdoor temperature, the recovered sensible heat would be about 5,878 BTU/h and the sensible heat loss in the exhausted air would be about 2,519 BTU/h.
For the R-2000 housing design, the amount of heat involved is quite small. Accordingly, for an integrated heating-ventilating-heat recovery system, emphasis should be on reduction of overall capital costs and elimination of freeze up problems associated with the operation of the air-to-air heat recovery system at freezing outdoor temperatures.
The proposed revision of the NBC calls for mechanical ventilation in all future houses regardless of the type of construction. It is anticipated that new energy efficient housing will have a considerably higher heat load requirement than houses built to the R-2000 standards. The requirement for ventilation and air-to-air heat recovery will remain the same as those for the R-2000 housing.
However, the substantially higher heat load requirement for this group of new houses compared to the R-2000 housing may require that different designs be used in the two different groups of houses, even if the general requirement for conditioning of the occupied space of the two types of houses is the same.
In the heating and ventilating field, there have been numerous proposals for improvements in heating and ventilating systems, and the following U.S. patents were considered in the preparation of this application:
Re. 17,577 (Dryssen) PA0 649,251 (Maude) PA0 1,214,047 (McGinnis) PA0 2,236,750 (Cross) PA0 2,274,341 (Mueller) PA0 2,497,184 (O'Brien) PA0 2,891,774 (Theoclitus) PA0 3,368,327 (Munters) PA0 3,756,310 (Becker) PA0 3,870,474 (Houston) PA0 4,034,482 (Briscoe) PA0 4,138,062 (Graden) PA0 4,227,375 (Tompkins) PA0 4,333,524 (Elkins) PA0 4,398,590 (Leroy) PA0 4,401,261 (Brown)